ASTM D7178-06

This practice provides a calculation method for determining the number of constrictions m of a non-woven geotextile (or of a layer of a composite material). This standard is not applicable to woven geotextiles, knitted geotextiles, heat-bonded geotextiles or any other type of geosynthetic.

The number of constriction represents the number of “windows” delimited by three or more fibers, in which soil particles could migrate. In that regard, it can be basically defined by the following equation: where t is the thickness and d_c the average distance between two constrictions. This value has been found to be relevant to explain the different filtration behaviors of non-woven geotextiles with similar opening sizes but different structures for various soil conditions (see Appendix X1 for details).

This value will be used in filtration research to evaluate the prediction of filtration efficiency and effectiveness of various non-woven geotextiles with similar opening sizes (Test Method D 6767).

Interpretation of the significance of m as calculated using this standard shall be done with care as some non-woven structures may not reflect the hypothesis used to establish the proposed equation (see Appendix X1 for details).

1.1 This practice describes the procedure used along with existing test method to determine the number of constrictions m of mechanically bonded non-woven geotextiles, based on thickness, mass per unit area and fiber properties

1.2 The number of constrictions is a property of non-woven geotextiles, which is complementary to opening size to predict their filtration behavior. It can be used to differentiate non-woven geotextiles with similar opening sizes but different structures (thickness, weight, fiber diameter, etc.). However, more research is needed to assess its significance when comparing two products with different opening sizes.

1.3 Consideration of the number of constriction is relevant in filtration applications where piping or clogging concerns are to be controlled with a high level of confidence, that is, for filters applications in critical soils.

1.4 This standard is for design purposes only and is not intended for quality control purposes.

This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.